WASHINGTON — The story line sounds familiar: a popular pain drug becomes a new way to get high as prescribing by doctors soars.

But the latest drug raising red flags is not part of the opioid family at the center of the nation’s drug epidemic. It’s a 25-year-old generic pill long seen as a low risk way to treat seizures, nerve pain and other ailments.

The drug, called gabapentin, is one of the most prescribed medications in the U.S., ranking ninth over the last year, according to prescription tracker GoodRx. Researchers attribute the recent surge to tighter restrictions on opioid painkillers, which have left doctors searching for alternatives for their patients.

Those same forces are changing the drugs that Americans abuse, according to experts.

“We’re basically squeezing people into other drugs because the prescription opioids are becoming a lot harder to get,” said Dr. Richard Dart, who tracks drug abuse through a national data network owned by the state of Colorado.

While prescriptions for opioids like Vicodin and Oxycontin have been falling since 2012, health regulators have seen increased overdoses with unexpected medications, including the over-the-counter diarrhea drug Imodium.

The Food and Drug Administration is now studying patterns of prescribing and illicit use of gabapentin and will soon share its findings, said Commissioner Scott Gottlieb.

“One of the lessons from this whole opioid crisis is that we probably were too slow to act where we saw problems emerging and we waited for more definitive conclusions,” Gottlieb said. “I don’t want to be sitting here five or 10 years from now lamenting that we didn’t take more aggressive action.”

Many doctors aren’t aware of gabapentin’s potential for abuse, particularly among those with a history of misusing drugs, said Rachel Vickers Smith of the University of Louisville.

People tracked in her research describe gabapentin as a “cheap high” that is almost “always available.” They report mixing the drug with opioids, marijuana and cocaine to enhance the high, with effects ranging from “increased energy” to a “mellow” numbness.

Medical journal articles estimate that between 15 and 25 percent of opioid abusers also use gabapentin. And emerging research suggests combining gabapentin and opioids heightens the overdose risks.

Gabapentin, on the market since 1993, has long been considered nonaddictive and is not tracked as closely as riskier drugs like opioids. But calls to U.S. poison control centers show a stark rise in abuse and overdoses.

The abuse rate increased nearly 400 percent between 2006 and 2015, according to poison center data analyzed by the RADARS research group within the Denver Health and Hospital Authority, a state-owned health system. The group’s work is funded by drugmakers and government agencies, though they don’t participate in the analysis or publication of the data.

In some parts of the U.S., the rise in gabapentin abuse has led to new restrictions and surveillance.

Last year, Kentucky became the first state to classify the drug as a “scheduled substance,” placing it among other high-risk medicines subject to extra restrictions and tracking. Gabapentin was detected in a third of fatal overdose cases analyzed by Kentucky medical examiners in 2016. Now, only health professionals registered with the federal government can prescribe the drug and patients are limited to five refills.

Ohio, Minnesota, West Virginia and several other states have begun tracking gabapentin through their prescription databases. Ohio took that step after gabapentin became the most dispensed drug in the state. State surveys of drug users also indicated it was “extremely easy to get” with a street price around $1.50 per capsule.

Alyssa Peckham, a researcher at Midwestern University in Arizona, believes a more comprehensive federal response is needed, including possibly reclassifying it nationwide. Like others, Peckham says gabapentin is not dangerous on its own, but can be when combined with opioids and other drugs that suppress breathing.

Still, there is little consensus about the next steps, or even the scope of the problem.

Michael Polydefkis, a neurologist at John Hopkins University who primarily treats seniors with nerve pain, says he has never seen patients deliberately misuse gabapentin.

And given recent restrictions on opioids by hospitals, insurers and government authorities, many physicians are wary of limiting any other medicines that can help treat pain. The Center for Disease Control and Prevention’s prescribing guidelines endorse gabapentin as a good choice for nerve pain.

But there are questions about how much is being prescribed for proven uses — and to what extent patients are benefiting. A recent review of research by the Cochrane Group confirmed gabapentin’s benefits for several forms of nerve pain, but found little evidence of its effectiveness for more common muscle and joint pain.

Historically, the vast majority of prescriptions have been for uses not OK’d by the FDA as safe or effective.

“This drug was kind of unusual in that it was prescribed as a kind of miracle pill that could be used for anything,” said Dr. Joseph Ross, a researcher at Yale University’s school of medicine.

In a recent Journal of the American Medical Association, he called for new studies of gabapentin’s real-world use.

The freewheeling prescribing dates to years of aggressive marketing by the drug’s original manufacturer, Warner-Lambert. The company pleaded guilty and agreed to pay more than $430 million in 2004 to settle charges that it promoted gabapentin for a slew of unapproved uses, including migraines, attention deficit disorder, bipolar disorder and Lou Gehrig’s disease. While doctors are free to prescribe drugs for any use, drugmakers can only market their products for those uses approved by the FDA.

Warner-Lambert was bought in 2000 by Pfizer, which continues to sell gabapentin under its original brand-name Neurontin. Pfizer also sells a similar drug named Lyrica, a blockbuster medication approved for fibromyalgia, diabetic nerve pain and several other uses. Unlike gabapentin, Lyrica is a scheduled substance under federal law, in part due to reports of euphoria and other side effects suggesting “abuse potential.”

With tighter restrictions and a lone manufacturer, Lyrica has not seen the same problems as gabapentin.

“Pfizer recognizes the importance of preventing the misuse and abuse of our medicines and will continue working with regulatory authorities and health officials to monitor the safety of these medicines,” the company said in a statement.